Automatic anti-skid brake control system

ABSTRACT

An anti-skid brake control system for a motor vehicle includes wheel sensors for producing signals representative of the rotational speed of each road wheel; an evaluation circuit responsive to the wheel speed signals for determining a reference speed that approaches the speed of the vehicle and for producing brake pressure control signals, in dependence upon the reference speed and the road wheel speeds; and a brake pressure controller responsive to the brake pressure control signals for varying the brake pressure applied to the road wheels. The evaluation circuit includes means for determining the vehicle speed from the average increase of the reference speed and this vehicle speed, so determined, is used to determine an increase in the value of the reference speed during periods of braking instability.

This application is a continuation of application Ser. No. 07/392,932,filed Aug. 7, 1989 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic anti-skid brake controlsystem for a motor vehicle. More particularly, the present inventionrelates to an anti-skid brake control system which includes wheelsensors for determining the speed of each individual vehicle wheel, anevaluation circuit to which the wheel speed sensor signals are appliedand which generates brake pressure control signals as a function of thewheel speeds, and brake pressure control valves, responsive to the brakepressure control signals, for varying the brake pressure at the vehiclewheels. In the evaluation circuit, the wheel speed signals are used toobtain at least one reference speed value that approximates the vehiclespeed characteristics so as to determine the slippage of any wheel. Suchslippage, together with the wheel speed signals and the vehicledeceleration, is used to control the brake pressure.

2. Prior Art

The German patent publication No. 2,313,763 to which U.S. Pat. No.3,907,378 corresponds discloses an automatic anti-skid brake controlsystem of the aforementioned type. This automatic anti-skid brakecontrol system, which also generates slippage signals and thereforerequires a reference speed, is provided with a vehicle decelerationsensor which is used to determine, in short repeated intervals, whetherthe braking effect of the vehicle is more efficient with or without theanti-skid control. In this case, the vehicle deceleration sensorconsequently serves to monitor the anti-skid control.

It is also known from the German patent publication No. 2,558,712, towhich U.S. Pat. No. 4,053,188 corresponds to formulate the referencespeed which is required for slippage control by referring to the speedsignals of two or more wheels included in the control system and toselect for reference speed formulation the most rapidly turning wheel.The wheel speed signals V_(R) trigger the possibly necessary slope ofthe reference speed signals either without modification or with at leastone prescribed slope. To decrease the reference speed V_(Ref) in case ofinstability (V_(R) <V_(Ref)), there are several slopes which can beactivated as a function of the determined rotational behavior of thewheel slope according to a prescribed algorithm.

SUMMARY OF THE INVENTION

It is an object of the invention to obtain a reference speed value foran Anti-skid brake control system with more simple means than inconventional systems while improving the match correlation to the actualvehicle speed characteristics.

This object, as well as other objects which will become apparent fromthe discussion that follows, are achieved, according to the presentinvention, by determining the vehicle deceleration from the averageslope of the reference speed value and by using this vehicledeceleration, so determined, to determine the slope of the referencespeed value during periods of braking instability.

It is of importance in the invention that the vehicle decelerationderived from the reference speed, in turn, affects the (negative) slopeof this reference speed in case of instability (V_(R) <V_(Ref)). Aparticularly favorable reference speed value formulation is achieved ifthe negative slope of the reference speed is selected during instabilityphases so as to be slightly higher (by a certain value or percentage ofe.g. 10-20%) than the vehicle deceleration; i.e., the reference speed isreduced slightly more than is the vehicle speed. The effects whichvehicle deceleration and slope of the reference speed have on each otherresult in a progressive change for both values. In particular, a higherdeceleration causes a greater slope of the reference speed and viceversa. Here, the reference speed can hardly surpass the vehicle speed.The value by which the negative slope of the reference speed is selectedto be greater than the calculated deceleration can be made a function ofthe pressure increase period, e.g. the number of the pressure increasepulses after a pressure drop (value increases with the number ofpulses). A positive change in coefficient of friction μ can thus be moreeasily registered and during long phases of instability on a low μ, theslope of the reference speed rises only slowly.

As mentioned above, a so determined vehicle deceleration can also beused for other control purposes; e.g., monitoring the control.

In addition to road conditions to which a μ slippage curve with adistinctive maximum corresponds, there are also road conditions wherethe friction coefficient μ constantly rises with an increasing slippage.As will be shown hereinafter, if the calculated vehicle deceleration hasreached values which are too high, the wheels will steadily increasetheir slippage in this case and finally lock. Since the vehicledeceleration was calculated too high, the pressure drop is tooinsensitive for the actual friction characteristics of the road. Thebrake pressure drops only to an extent such that the circumferentialdeceleration of the wheel does not continue to increase.

However, in most of the cases, this does not suffice to significantlyreaccelerate the wheel. Therefore, in order to achieve a sufficientlyhigh wheel acceleration, the calculated vehicle deceleration must becorrected such that it conforms with the actual vehicle deceleration.

In order to achieve this correction, an improvement of the inventioncompares the calculated vehicle deceleration with the deceleration ofone or several wheels and prevents the subsequent pressure build-up atthe moment when the calculated vehicle deceleration surpasses the wheeldeceleration. Since a stable running wheel cannot have a higherdeceleration than the vehicle deceleration, such a condition ispractically impossible and, if it occurs, it must have been caused by amiscalculation of the vehicle deceleration. Due to the undesiredpressure drop the wheel does not reenter into a higher slippage. Thereference speed runs against the wheel circumferential speed and issupported by the latter. The calculated vehicle deceleration is thuscorrected towards a value approaching the actual vehicle deceleration.

Today it is very common to equip vehicles with a so-called emergencywheel as a spare wheel. This emergency wheel is usually smaller indiameter and consequently rotates faster than a normal wheel. Includingthis speed in the reference value is formed--especially if maximum valueformation is done-- results in a false reference value which causesunderbraking.

In accordance with the improvement of the invention, a supplementaryreference speed is formed in addition to the reference speed so as toavoid this disadvantage. For the formation of this supplementaryreference speed, the most rapidly rotating wheel determines theincrease. The vehicle deceleration is derived from this supplementaryreference speed as described below.

However, for the formation of the actual reference speed for theslippage, the value thereof is determined by the second fastest wheel.As compared to this, the vehicle deceleration derived from thesupplementary reference speed is used to determine the slope duringinstability phases.

Therefore, it is here possible to derive a reference speed from thespeed of each individual wheel and to only manipulate the slope thereofduring the instability phases by means of the vehicle decelerationderived from the supplementary reference speed.

For a full understanding of the present invention, reference should nowbe made to the following detailed description of the preferredembodiments of the invention and to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an anti-skid brake control system of thetype to which the present invention relates.

FIG. 2 is a block diagram of a portion of the evaluation circuit of FIG.1 which provides the reference speed value in accordance with theinvention.

FIGS. 4-6 and 8 are explanatory time diagrams illustrating the operationof the anti-skid brake control system of the present invention.

FIGS. 7 and 9 are block diagrams of circuits which provide alternativestoo the embodiment of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the components of an anti-skid brake control system.Reference numerals 1-4 designate the four speed sensors, assigned to thefour vehicle wheels, which determine the wheel speeds.

An evaluation circuit to which the speed signals of the sensors 1-4 areapplied is indicated by block 5 and four solenoid valves for brakepressure variation, which are activated by brake pressure controlsignals generated by the evaluation circuit 5, are indicated by theblocks 6-9. An OR-gate 10 included in the circuit of FIG. 1 will bediscussed hereinafter.

The evaluation circuit 5 generates slippage signals s₁ to s₄ in additionto other signals. These slippage signals are used in the activatinglogic of the circuits for the formation of the activating signals forvalves 6 to 9. The slippage signals s₁ to s₄ are derived from the speedsignals from the sensors 1 to 4 assigned to these wheels. These speedsignals are designated as V_(R1), V_(R2), V_(R3), and V_(R4),respectively.

FIG. 2 illustrates a possible circuit for slippage signal formation.

In this circuit, the highest value V_(max) is filtered out of the fourwheel speed signals V_(R1) to V_(R4) in a unit 20 and supplied to a unit21 where the momentary reference speed V_(Ref) is generated. Toaccomplish this, the previously determined reference speed value, whichis stored in a unit 22, is also required. Furthermore, in case ofinstability, the vehicle deceleration a_(f) is also required which issupplied via line 23'. Finally, a value is entered into a unit 21 viaunit 24 which determines by how much (by what percentage) the slope ofthe reference speed must be increased with respect to the vehicledeceleration. A value T_(A) supplied via terminal 24' varies thepercentage as a function of the brake pressure build-up time.

A change value Δ_(V) is generated in unit 21 such that, based on thereference speed value determined in the preceding cycle time, in casethe value V_(max) surpasses this reference, the reference speed value isincreased according to the value V_(max), and in the case of instability(V_(max) <V_(Ref)), the reference speed value is reduced according tothe vehicle deceleration a_(f) supplied via line 23', considering thepercentage determined by the unit 24.

The difference Δ_(V) between successively determined reference speedvalues is supplied to a unit 23 as well as to a unit 22 which correctsthe old reference value. The momentarily present reference speed valueV_(Ref) is supplied to a unit 25 to which the wheel speed signals V_(R1)to V_(R4) are also applied so as to generate the slippage values s₁ tos₄.

The vehicle deceleration a_(F) is generated in the unit 23 and suppliedto the unit 21. This generation of a_(F) can be carried out in differentways: Several successively generated Δ_(V) values determined during acycle time T can be summed up and output as vehicle deceleration at theend of this time T. In order to improve the measurements, differentcycle times can be employed such that different cycles times are usedfor adding and, if necessary, different vehicle decelerations aresuccessively output. This permits using a shorter cycle times when thecontrol is not employed.

However, it is also possible to provide the unit 23 with a number ofmemories where the Δ_(V) values can be successively stored. Every newΔ_(V) value is then substituted in place of the oldest stored value. Anew vehicle deceleration can be output after each newly stored Δ_(V)value (cycle time of the Δ_(V) calculation), thereby avoiding waitinguntil the end of a cycle time t which is a multiple of the cycle timesT.

It is also possible to sum up in unit 23 all incoming Δ_(V) values, todivide the total of the values by a prescribed value then to deduct theresult thereof from the previously determined total, and to output thedifference as the new vehicle deceleration. In this case, too, theresult is one new vehicle deceleration value per interval of the cycletime T (of the processor).

Further, it is indicated in FIG. 2 that a brake pressure drop can beprevented if the wheel deceleration is smaller than the vehicledeceleration.

For this purpose, the wheel speed signal is differentiated (unit 26) andcompared to the vehicle deceleration in unit 27. If the vehicledeceleration signal surpasses the wheel deceleration signal, thecomparator 27 produces a signal which is supplied via a terminal 28 tothe terminal 11 and to the OR-gate 10 of FIG. 1.

From the diagram of FIG. 3, in which the reference speed V_(Ref), thewheel speed V_(R) and the calculated vehicle deceleration a_(F) areindicated over the time, it is possible to see the progressiverelationship between vehicle deceleration and the slope of the referencespeed. In a slippage curve, as represented in FIG. 5, it is possible,that the calculated vehicle deceleration a_(F) increases, as representedin FIG. 4b, thus resulting in a change of the reference value V_(Ref)such that the latter more and more departs from the vehicle speed V_(R1)as shown in FIG. 4a. In this case, the wheels will lock despite theanti-lock braking system. However, the above described blocking of thepressure build-up during a vehicle deceleration higher than the wheeldeceleration causes behavior such as that represented in FIG. 6a. Theunblocked parameters are indicated in dotted lines. Before t₁ in theillustrated version the brake pressure build-up is blocked such that thewheel speed V_(R) does not further decrease and supports the referencevalue V_(ref) after t₁. FIG. 6b shows the corresponding characteristicof the vehicle deceleration a_(f).

The embodiment in FIG. 7 operates with a supplementary reference value.In a unit 70 the maximum value is again formed from the wheel speedsV_(R1) and V_(R2) and this maximum value is supplied to a unit 71 thatassumes the functions of units 21 and 23; this unit 71 consequentlyoutputs the slope of the determined reference speed and supplies thisslope to a unit 73.

Proceeding on the assumption that the wheel speeds V_(R1) and V_(R2) areincluded in the formation of the reference value, the unit 72, selectingthe respectively smaller value, can determine the second fastest wheelspeed. In the unit 73, which corresponds to units 21 and 22 of FIG. 2,the actual speed reference value (terminal 74) is determined based onthe calculated vehicle deceleration (from unit 71) and the percentage(terminal 75). FIG. 8 shows the corresponding diagram for the emergencywheel. The wheel R₁ is the emergency wheel which is smaller in diameterthan the normal wheels. V_(R1) is the characteristic curve for thiswheel and in case a wheel locking is likely to occur, the supplementaryreference value V'_(ref) can take effect. V_(R1), and partiallyV'_(Ref), determine the vehicle deceleration speed which is used todetermine the slope of the actual reference speed value V_(ref) in thecase where V_(R) <V_(Ref).

FIG. 9 illustrates again the formation of a supplementary referencespeed within a unit 81 from the maximum values of the wheel speedsV_(R1) and V_(R2) (unit 80) which affords the basis to determine thevehicle deceleration a_(F). This deceleration is used by a unit 82 todetermine the slope of the reference speed value V_(Ref) in the casewhere V_(R) <V_(Ref). This reference speed value serves only for theslippage formation at the wheel R₁.

There has thus been shown and described a novel automatic anti-skidbrake control system which fulfills all the objects and advantagessought therefor. Many changes, modifications, variations and other usesand applications of the subject invention will, however, become apparentto those skilled in the art after considering this specification and theaccompanying drawings which disclose the preferred embodiments thereof.All such changes, modification, variations and other uses andapplications which do not depart from the spirit and scope of theinvention are deemed to be covered by the invention which is limitedonly by the claims which follow.

We claim:
 1. In an anti-skid brake control system for a motor vehiclehaving a plurality of vehicle wheels, said system comprising, incombination:(1) wheel speed sensors for producing wheel speed signalsrepresentative of the rotational speeds of the respective vehiclewheels; (2) evaluation circuit means, responsive to the wheel speedsignals of said sensors, for determining a reference speed thatapproximates the speed of said vehicle, and for producing brake controlsignals in dependence upon slippage values generated from said referencespeed and said wheel speed signals; and (3) brake pressure controlmeans, responsive to said brake pressure control signals, for varyingthe brake pressure applied to said vehicle wheels; the improvementwherein said evaluation circuit means includes means for determining thevehicle deceleration from change values between several successivelydetermined reference speeds, and wherein said vehicle deceleration, sodetermined, is used to define the slopes of said reference speed duringperiods of braking instability.
 2. The anti-skid brake control system inaccordance with claim 1, wherein said slope of said reference speedduring the periods of instability is modified to be higher than thevalue of deceleration of said vehicle by a prescribed amount.
 3. Ananti-skid brake control system in accordance with claim 1, wherein saidvehicle deceleration determining means includes digital circuit meansfor summing successively determined changes Δ_(V) of said referencespeed in successive periods of time in accordance with their sign.
 4. Ananti-skid brake control system in accordance with claim 1, wherein saidvehicle deceleration determining means includes digital circuit meansfor storing a plurality n of successively determined change values Vbetween successively determined reference speeds, such that the oldeststored value is deleted upon storage of a new change value(n+1), saidvehicle deceleration determining means including means for summing thestored change values V together after each new change value is stored.5. An anti-skid brake control system in accordance with claim 1, whereinsaid vehicle deceleration determining means includes digital circuitmeans for determining the sum of successively determined change valuesof said reference speed; means for dividing said sum, so determined, bya prescribed factor to obtain a quotient; and means for subtracting saidquotient from said sum to obtain a remainder, wherein the vehicledeceleration is set equal to said remainder.
 6. An anti-skid brakecontrol system in accordance with claim 1, wherein said evaluationcircuit means determines the vehicle deceleration from the sum of thechange values which occur between decreases in vehicle acceleration. 7.An anti-skid brake control system in accordance with claim 2, whereinthe value by which the slope of the reference speed is higher than thevehicle deceleration is a function of a brake pressure build-up valueafter a pressure drop.
 8. An anti-skid brake control system inaccordance with claim 1, wherein the calculated vehicle deceleration iscompared to the deceleration of a vehicle wheel and a brake pressurebuild-up is inhibited if the wheel deceleration is smaller than thecalculated vehicle deceleration.
 9. An anti-skid brake control system inaccordance with claim 1 whereinthe reference speed for determining theslippage is generated from the speed of the second fastest rotatingwheel, a supplementary reference speed is generated from the speed ofthe fastest rotating wheel, and the vehicle deceleration is determinedfrom change values between several successively determined values of thesupplementary reference speed, and said vehicle deceleration, sodetermined, is used to determine the slope of said reference speedduring said periods of braking instability.